Electrophotographic image forming apparatuses have been used for offices. Because of being able to easily perform on-demand image formation, electrophotographic image forming apparatuses are recently used for commercial printing. The electrophotographic image forming apparatuses for use in commercial printing are required to fulfill the following requirements 1-5:
1. Higher speed printing;
2. Larger quantity printing;
3. Higher image qualities;
4. Image formability on various kinds of recording materials; and
5. Lower running costs.
In order to fulfill the requirements 1, 2 and 5, it is necessary to prolong the life of photoreceptors, which are main devices of such electrophotographic image forming apparatus. Photoreceptors are broadly classified into inorganic photoreceptors typified by photoreceptors using amorphous silicon, and organic photoreceptors typified by photoreceptors using an organic charge generation material and an organic charge transport material. Organic photoreceptors have the following advantages over inorganic photoreceptors:
1. Good optical properties such as wide light absorption range, and large absorbance;
2. Good electric properties such as high sensitivity and good charge stability;
3. Wide material selectivity;
4. Good productivity;
5. Low costs; and
6. Little toxicity.
However, organic photoreceptors have the following disadvantages:
1. The scratch resistance is poor, thereby easily forming defective images; and
2. The abrasion resistance is poor, thereby easily causing deterioration of sensitivity and charging ability, and leak of charges, resulting in formation of abnormal images such as low density images and images with background fouling.
In attempting to improve the scratch resistance and the abrasion resistance, several photoreceptors have been proposed in which a layer having a good mechanical strength is formed as an outermost layer of a conventional organic photoreceptor.
For example, a photoreceptor is proposed which has a photosensitive layer including a crosslinked material obtained by crosslinking a positive hole transport compound having two or more chain polymerizable groups in a molecule. In addition, photoreceptors have been proposed in which a protective layer obtained by irradiating a composition, which includes a radically polymerizable charge transport compound, a tri- or more-functional radically polymerizable monomer, and a photo-polymerization initiator, with ultraviolet rays to crosslink the composition. These photoreceptors have good scratch resistance and abrasion resistance while exhibiting high stability to withstand environmental conditions. Therefore, high quality images can be produced without using a drum heater to heat the photoreceptors to reduce the moisture content of the photoreceptors.
Further, a photoreceptor is proposed in which an ultraviolet absorbent is included in a protective layer, which is formed on a photosensitive layer by forming a layer and irradiating the layer with ultraviolet rays to crosslink the layer, to prevent the electric properties of the photosensitive layer from being deteriorated by the ultraviolet rays.
It can be understood from these proposals that by forming a three dimensionally crosslinked protective layer on a photoreceptor by crosslinking a radically polymerizable charge transport material (particularly charge transport material having an acrylic group) together with an optional acrylic monomer, a good combination of scratch resistance, abrasion resistance and electric properties can be imparted to the photoreceptor. Therefore, such a photoreceptor may be used for commercial printing. However, in commercial printing, the requirements for image qualities become severer and severer recently. Therefore, it is necessary for a photoreceptor used for commercial printing to reduce variation in potential after repeated use (hereinafter this variation is sometimes referred to as potential variation with time) and variation in potential from place to place on the surface of the charged photoreceptor (hereinafter this variation is sometimes referred to as in-plane potential variation). The above-mentioned photoreceptors are not satisfactory with respect to this point.
The reason therefor is considered to be as follows. Specifically, in order to form a protective layer having a high cross-linkage density by performing a radical polymerization reaction, a method in which a coating liquid including such a radically polymerizable charge transport compound and a photo-decomposable radical polymerization initiator is applied, followed by irradiation of light (ultraviolet rays), or a method in which a coating liquid including such a radically polymerizable charge transport compound is applied, followed by irradiation of electron beams or radiation beams to directly excite an acrylic group of the compound is used. In any of these methods, the charge transport material in the protective layer is excited and thereby part of the compound is decomposed, resulting in deterioration of the charge transport property of the photoreceptor.
In attempting to solve the problem, there is a proposal in that an ultraviolet absorbent is included in the protective layer. However, addition of an ultraviolet absorbent to a protective layer deteriorates the charge transport property of the photoreceptor, and inhibits a radical polymerization reaction, thereby making it impossible to form a protective layer having a high cross-linkage density. In addition, there is a singlet oxygen quencher (such as nickel dithiorate complexes), which can inhibit decomposition of a dye. Addition of such a quencher to a protective layer perfectly deteriorates the photosensitivity of the photoreceptor.
Thus, there is no photoreceptor which has a crosslinked protective layer obtained by crosslinking a radically polymerizable charge transport compound using ultraviolet rays and which can produce high quality images (i.e., little image density variation with time and little in-plane image density variation) so as to be used for commercial printing use.
For these reasons, the inventors recognized that there is a need for a photoreceptor which has a protective layer having a good combination of charge transport property, scratch resistance, and abrasion resistance and which can produce higher quality images than ever.